When you have a job you are working for an employer and you are getting paid a certain amount for your services.  If you don't have experience in a certain area but you want to learn in the real work environment you can get an internship.  If you have an internship (you are called an "intern") you are working for an employer and in many cases you are not getting paid.  There are some paid internships, but in general an internship is about learning, not about making a lot of money.  Internships usually are short term, lasting from a few months to about a year.